Counter ICs

A Shift Register is a sequential logic device made up of a cascade of flip-flops, through which a sequence of bits are ’shifted’. These devices are commonly used to convert between serial and parallel interfaces.

How it works – basic example

In a Shift Register the output of each flip-flop is connected to the input of the next flip-flop in the cascade. With each cycle of the connected clock, the bits are ’shifted’ down the cascade by one flip-flop.

A simple analogy to understand the basic theory or operation would be to consider a glass tube with an opening both ends, and placing a ball marked ’0’ or ’1’ into one end of the tube at a regular intervals.

As each ball is placed into the tube, all the balls are ’shifted’ along by one position. The sequence in which the balls marked ’0’ and ’1’ where fed in is preserved. Also each time one ball is fed in, the ball that has been in the cascade the longest drops out. So if the tube can contain 4 balls at any one time, this could be described as a 4-bit Shift Register.

Common Types of Shift Register

• Serial-in serial-out (SISO) – In this type of Shift Register, the data input and output is serial. With each clock cycle a new bit is fed into the cascade, with the bit furthest along the cascade exiting as an output. So in a 4-bit register, the output trails the input by 4 clock cycles. This matches the simple example above, with the ball that exits the tube being the output.

• Serial-in parallel-out (SIPO) – In this configuration data is converted from serial into parallel format. As described for the SISO variant above, data is fed in serially and shifted down the cascade, the difference is that each flip-flop has an output, allowing the entire bit array to be output simultaneously as a parallel output. This may be done using a latched of buffered output.

• Parallel-in serial-out (PISO) – This variant takes a parallel input and converts into a serial output. To do this the shift register must implement a write/shift sequence. The parallel data input is (written) into each flip-flop of the array simultaneously in one clock cycle, then for the next cycles the register goes into shift mode as the bits are shifted down the flip-flop cascade, being output as serial data. The process then repeats.

• Parallel-in parallel-out (PIPO) – This type of register differs slightly from the others in that inputs and outputs of each flip-flop in the array are not interconnected. Each flip-flop takes one bit of the parallel input, stores it for a cycle, and then outputs the data simultaneously with the other flip-flops in the register, creating a parallel output that matches the input. Like the SISO type above, PIPO variants can be used to temporarily store data, or act a time delay device.

• Universal Shift Register – A configurable Shift Register that can be programmed to act as a SISO, SIPO, PISO or PIPO device, often on the fly.

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Description Price Package Type Logic Function Number of Stages Logic Family Mounting Type Counter Type Operation Mode Number of Elements Pin Count Minimum Operating Supply Voltage Maximum Operating Supply Voltage Dimensions Length Width
RS Stock No. 806-1684
Mfr. Part No.MC74HCT595ADG
Each (In a Pack of 48)
SOIC Shift Register 8 MC74 Surface Mount - Serial/Parallel 1 16 4.5 V 5.5 V 10 x 4 x 1.5mm 10mm 4mm
RS Stock No. 163-0598
Mfr. Part No.MC74HC4020ADR2G
Each (On a Reel of 2500)
SOIC Counter 14 MC74 Surface Mount Binary Up Counter 1 16 2 V 6 V 10 x 4 x 1.5mm 10mm 4mm
RS Stock No. 145-2980
Mfr. Part No.MC14557BDWG
Each (In a Tube of 47)
SOIC W Shift Register 64 CMOS Surface Mount - Serial 1 16 -0.5 V 18 V 10.45 x 7.6 x 2.4mm 10.45mm 7.6mm
RS Stock No. 663-0127
Mfr. Part No.CD74HC191E
Each (In a Pack of 5)
PDIP Counter 4 HC Through Hole Binary Down Counter, Up Counter 1 16 2 V 6 V 19.3 x 6.35 x 4.57mm 19.3mm 6.35mm
RS Stock No. 709-2485
Mfr. Part No.CD40103BE
Each (In a Pack of 5)
PDIP Counter 8 4000 Through Hole Binary Down Counter 1 16 3 V 18 V 19.3 x 6.35 x 4.57mm 19.3mm 6.35mm
RS Stock No. 663-2155
Mfr. Part No.SN74HC595DW
Each (In a Pack of 10)
SOIC Shift Register 8 HC Surface Mount - Serial to Serial/Parallel 1 16 2 V 6 V 10.28 x 7.52 x 2.35mm 10.28mm 7.52mm
RS Stock No. 145-1120
Mfr. Part No.CD74HC595DW
Each (In a Tube of 40)
SOIC Shift Register 8 HC Surface Mount - Serial to Parallel 1 16 2 V 6 V 10.5 x 7.6 x 2.35mm 10.5mm 7.6mm